[client] renderer/egl: implement support for rendering help text

The help text is rendered in the bottom left corner on a semi-transparent
background, very similar to how the FPS text is rendered.

client/renderers/EGL/shader/help_bg.frag

@@ -0,0 +1,8 @@
+#version 300 es
+
+out highp vec4 color;
+
+void main()
+{
+  color = vec4(0.0, 0.0, 1.0, 1.0);
+}